Citrulline, not an amino acid that constitutes a protein in a living organism, is one of the intermediates in the urea cycle, and is produced from arginine along with nitrogen oxide (NO), which is known as a substance possessing vasodilating action, and is condensed with aspartic acid and regenerated into arginine. Citrulline is known to exhibit useful actions such as ammonia metabolism promotion (Non-patent Document 1), blood flow improvement by vasodilation (Non-patent Document 2), blood pressure reduction (Non-patent Document 3), neurotransmission (Non-patent Document 4), immunopotentiation (Non-patent Document 5), and active oxygen elimination (Patent Document 1). For this reason, with the expectation for these actions, it is common practice to take citrulline in the form of pharmaceuticals, functional foods and the like.
It is known that in aqueous solutions of basic amino acids such as lysine, arginine, ornithine, hydroxylysine, and histidine, bad odors are generated due to storage or heating, and that the bad odors are suppressed by adding a water-soluble reducing agent and a chelating agent to the aqueous solutions (Patent Document 2).    patent document 1: JP-A-2002-226370    patent document 2: JP-A-5-246962    non-patent document 1: Cell Biochemistry & Function, 2003, vol. 21, p. 85-91    non-patent document 2: European Journal of Pharmacology, 2001, vol. 431, p. 61-69    non-patent document 3: Journal of Clinical Investigation, 1991, vol. 88, p. 1559-1567    non-patent document 4: Gastroenterology, 1997, vol. 112, p. 1250-1259    non-patent document 5: The journal of biological chemistry, 1994, vol. 269, p. 9405-9408